This invention relates to a heat pump having enhanced performance characteristics when operating in the heating mode.
As disclosed in U.S. Pat. No. 6,131,402 to Mills et al., many heat pumps when operating in a heating mode tend to experience a phenomenon that is generally referred to as cold blow. During a cold blow period the supply air temperature exiting the indoor coil falls below a determinable set point and a person situated in the indoor comfort zone has an uncomfortable sensation that cold air is being discharged from the heat pump. This occurs primarily because most heat pumps are optimized for operating in the cooling mode rather than the heating mode. The fan servicing the indoor coil is typically set at one speed that is optimized for the cooling mode of operations. This set fan speed, however, is relatively high and creates an indoor airflow that is greater than that required for optimum heat transfer particularly when the outdoor temperature falls below 50° F.
As explained in further detail in the above noted patent, Mills et al. the outdoor or ambient temperature is monitored and the speed of the indoor fan is reduced periodically as the outdoor temperature falls so as to maintain the indoor supply temperature at a level desired. Three different indoor airflow rates are employed with each flow rate covering a discrete outdoor temperature range. However, as the outdoor temperature decreases within each range temperature the indoor temperature will correspondently decrease.
Although, the heat pump system described in the Mills et al. patent provides greater comfort to the occupants of the comfort zone when operating in the heating mode, the changes in the supply airflow forces the system compressor to operate in varying pressure ranges which can lead to a reduction in compressor stability and reliability. The heat pump system disclosed in the Mills et al. patent also requires relatively steep and abrupt changes in the indoor fan speed. This can also lead to a reduction in fan stability and produce disturbing fan noise levels.
When a heat pump cycle is reversed from a cooling mode of operation to a heating mode, the low pressure refrigerant vapor line becomes a high pressure discharge vapor line and there is a danger that the pressure in the line can exceed the maximum allowable line pressure for the particular heat pump unit. Some heat pumps, and in particular those employing a R-410a refrigerant, are equipped with vapor pressure switches which are arranged to inactivate the outdoor coil fan when the vapor pressure approaches the allowable minimum level. This cycling of the outdoor coil fan can adversely effect the supply air temperature as well as the system capacity and efficiency.